Interactions between apposed poly(L-glutamic acid) monolayers in water were investigated by direct surface force measurements. The average degree of polymerization was 21. Surface force profiles consisted of a long-range electrostatic repulsion and a short-range steric repulsion. Secondary structures of the polypeptide were found to determine the short-range steric repulsion. The elastic compressibility moduli of the polypeptide monolayers were obtained to be 5.6 +/- 2.2 MPa for the beta(2)-structure-rich layer and 0.2 +/- 0.1 MPa for the layer of ionized extended chains.